Will the C# implementation support proto2 messages?
What is the capability story? I assume the wire format is compatible if no
proto3 exclusive features are used?

On Tuesday, August 4, 2015 at 5:43:36 AM UTC-7, Jon Skeet wrote:
> That looks like you're expecting a protobuf.net-style approach - to which
> the answer is "no" and will continue to be "no".
>
> The C# support will continue to be based on generated code, but there's a
> new code generator and runtime now in the master branch. The main changes
> from the previous code are:
>
> - proto3-only support (no proto2 at all)
> - mutable generated types rather than the Java-style builders and
> immutable messages
>
> Jon
>
> On Monday, 3 August 2015 22:50:06 UTC+1, The Nguyen Xuan wrote:
>>
>> Does this version support object type in C# ?
>>
>> ex:
>>
>> [ProtoMember(1)]
>> public object A {get;set;}
>>
>> thank.
>>
>> Vào 11:51:01 UTC+7 Thứ Năm, ngày 11 tháng 12 năm 2014, Feng Xiao đã viết:
>>>
>>> Hi all,
>>>
>>> I just published protobuf v3.0.0-alpha-1 on our github site:
>>> https://github.com/google/protobuf/releases/tag/v3.0.0-alpha-1
>>>
>>> This is the first alpha release of protobuf v3.0.0. In protobuf v3.0.0,
>>> we will add a new protobuf language version (aka proto3) and support a
>>> wider range of programming languages (to name a few: ruby, php, node.js,
>>> objective-c). This alpha version contains C++ and Java implementation with
>>> partial proto3 support (see below for details). In future releases we will
>>> add support for more programming languages and implement the full proto3
>>> feature set. Besides proto3, this alpha version also includes two other new
>>> features: map fields and arena allocation. They are implemented for both
>>> proto3 and the old protobuf language version (aka proto2).
>>>
>>> We are currently working on the documentation of these new features and
>>> when it's ready it will be updated to our protobuf developer guide
>>> <https://developers.google.com/protocol-buffers/docs/overview>. For the
>>> time being if you have any questions regarding proto3 or other new
>>> features, please post your question in the discussion group.
>>>
>>> CHANGS
>>> =======
>>> Version 3.0.0-alpha-1 (C++/Java):
>>>
>>> General
>>> * Introduced Protocol Buffers language version 3 (aka proto3).
>>>
>>> When protobuf was initially opensourced it implemented Protocol
>>> Buffers
>>> language version 2 (aka proto2), which is why the version number
>>> started from v2.0.0. From v3.0.0, a new language version (proto3) is
>>> introduced while the old version (proto2) will continue to be
>>> supported.
>>>
>>> The main intent of introducing proto3 is to clean up protobuf before
>>> pushing the language as the foundation of Google's new API platform.
>>> In proto3, the language is simplified, both for ease of use and to
>>> make it available in a wider range of programming languages. At the
>>> same time a few features are added to better support common idioms
>>> found in APIs.
>>>
>>> The following are the main new features in language version 3:
>>>
>>> 1. Removal of field presence logic for primitive value fields,
>>> removal
>>> of required fields, and removal of default values. This makes
>>> proto3
>>> significantly easier to implement with open struct
>>> representations,
>>> as in languages like Android Java, Objective C, or Go.
>>> 2. Removal of unknown fields.
>>> 3. Removal of extensions, which are instead replaced by a new
>>> standard
>>> type called Any.
>>> 4. Fix semantics for unknown enum values.
>>> 5. Addition of maps.
>>> 6. Addition of a small set of standard types for representation of
>>> time,
>>> dynamic data, etc.
>>> 7. A well-defined encoding in JSON as an alternative to binary
>>> proto
>>> encoding.
>>>
>>> This release (v3.0.0-alpha-1) includes partial proto3 support for
>>> C++ and
>>> Java. Items 6 (well-known types) and 7 (JSON format) in the above
>>> feature
>>> list are not implemented.
>>>
>>> A new notion "syntax" is introduced to specify whether a .proto file
>>> uses proto2 or proto3:
>>>
>>> // foo.proto
>>> syntax = "proto3";
>>> message Bar {...}
>>>
>>> If omitted, the protocol compiler will generate a warning and
>>> "proto2" will
>>> be used as the default. This warning will be turned into an error in
>>> a
>>> future release.
>>>
>>> We recommend that new Protocol Buffers users use proto3. However, we
>>> do not
>>> generally recommend that existing users migrate from proto2 from
>>> proto3 due
>>> to API incompatibility, and we will continue to support proto2 for a
>>> long
>>> time.
>>>
>>> * Added support for map fields (implemented in C++/Java for both
>>> proto2 and
>>> proto3).
>>>
>>> Map fields can be declared using the following syntax:
>>>
>>> message Foo {
>>> map<string, string> values = 1;
>>> }
>>>
>>> Data of a map field will be stored in memory as an unordered map and
>>> it
>>> can be accessed through generated accessors.
>>>
>>> C++
>>> * Added arena allocation support (for both proto2 and proto3).
>>>
>>> Profiling shows memory allocation and deallocation constitutes a
>>> significant
>>> fraction of CPU-time spent in protobuf code and arena allocation is a
>>> technique introduced to reduce this cost. With arena allocation, new
>>> objects will be allocated from a large piece of preallocated memory
>>> and
>>> deallocation of these objects is almost free. Early adoption shows
>>> 20% to
>>> 50% improvement in some Google binaries.
>>>
>>> To enable arena support, add the following option to your .proto
>>> file:
>>>
>>> option cc_enable_arenas = true;
>>>
>>> Protocol compiler will generate additional code to make the generated
>>> message classes work with arenas. This does not change the existing
>>> API
>>> of protobuf messages and does not affect wire format. Your existing
>>> code
>>> should continue to work after adding this option. In the future we
>>> will
>>> make this option enabled by default.
>>>
>>> To actually take advantage of arena allocation, you need to use the
>>> arena
>>> APIs when creating messages. A quick example of using the arena API:
>>>
>>> {
>>> google::protobuf::Arena arena;
>>> // Allocate a protobuf message in the arena.
>>> MyMessage* message = Arena::CreateMessage<MyMessage>(&arena);
>>> // All submessages will be allocated in the same arena.
>>> if (!message->ParseFromString(data)) {
>>> // Deal with malformed input data.
>>> }
>>> // Must not delete the message here. It will be deleted
>>> automatically
>>> // when the arena is destroyed.
>>> }
>>>
>>> Currently arena does not work with map fields. Enabling arena in a
>>> .proto
>>> file containing map fields will result in compile errors in the
>>> generated
>>> code. This will be addressed in a future release.
>>> =======
>>>
>>> Thanks,
>>> Feng
>>>
>>
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